1. Field of the Invention
The invention relates to solar cells for converting light into electrical energy and more particularly to separate panels of solar cells stacked one upon the other to form a stacked solar cell module.
As used throughout this specification and the claims the following terms have the following meanings:
Solar cell or cell, an individual discrete member having a junction therein and capable of directly converting photons to electrical energy;
Thin film solar cell, a solar cell fabricated from microcrystalline, amorphous, compound, or semiconductor material other than single crystal semiconductor material deposited in situ upon a substrate;
Panel, an array or group of solar cells interconnected to provide an output of electrical energy;
Module, one or more panels confined within an appropriate housing and capable of being placed in long term service for production of electrical energy.
Array, depending upon the context, a group of solar cells forming a panel or a group of modules positioned to receive photons for direct conversion to electrical energy.
Spectral response, sensitivity to a predetermined portion of the light spectrum less than the whole thereof.
2. The Prior Art
It has long been desirable to capture as much of the sun's spectrum as possible to convert it directly into electrical energy through the utilization of solar cells. Conventional single-crystal solar cells appear to be rapidly approaching the ultimate intrinsic limits of their conversion efficiency. As a result, other types of solar cells are being considered and constructed, such as those made from gallium arsenide and other similar materials. While such materials may have a higher efficiency of conversion than single-crystal silicon, there is a limit to the ultimate efficiency which can be expected.
To increase the collection efficiency, consideration has been given to cascading solar cells, as is discussed in the article "Material and Device Considerations for Cascade Solar Cells" by Salahm Bedair, Sunil B. Phatak and John R. Hauser which was published in the April 1980 issue of IEEE Transactions on Electron Devices, Volume ED-27, No. 4, pp. 822-831. As is therein disclosed, one of the approaches to improve efficiency makes use of two or more cells to more efficiently utilize the solar spectrum. A first approach utilizing such plurality of cells is that of spectrum splitting. That is, the solar spectrum is split into two or more parts by the use of filters and as a result a narrower band of photon energies is incident on each individual cell. As a result, each cell must respond to a narrower range of photon energies and each of the cells can then be optimized at a higher efficiency than can one single cell for the entire solar spectrum. One experimental apparatus used a silicon single-crystal cell for the low energy photons and an aluminum gallium arsenide (AlGaAs) cell for the high energy photons.
Another approach is to connect two individual solar cells in optical and electrical series. In this approach the wide bandgap cell is located above the narrow bandgap cell. The high energy photons are then absorbed in the wide bandgap top cell while the low energy photons (those below the bandgap of the top cell) pass to the bottom cell for absorption. The cascaded cells were formed by utilizing a monolithic structure using a heavily doped tunneling interface to interconnect the cascaded cells. Such was accomplished by using an aluminum gallium arsenide/gallium arsenide cell structure with a heavily doped, very thin tunneling interface layer having a large bandgap (as large or larger than that of the top cell).
It will readily be recognized that the spectrum splitting concept requires mirrors, filters and two distinct solar cells. In addition, two distinct packages housing those solar cells and the spectrum splitting device are required. Those skilled in the art will readily recognize that the utilization of such a concept for commercial applications is not cost effective as compared to the state of the art solar cells.
The cascading of solar cells by optical and electrical series connection through the utilization of the thin highly doped tunneling interface requires matching of short circuit currents in order to achieve proper operation. This matching of short circuit currents becomes impossible when the cell is exposed to ambient sunlight simply because the frequencies of the ambient light on earth change throughout the day as the sun moves across the sky. Thus it will be recognized by those skilled in the art that short circuit current matching cannot be accomplished except for one frequency of the spectrum. Furthermore, if single-crystal structures are to be used for the cascaded cells as disclosed in the prior art, the interface connections require lattice matching to obtain the appropriate tunneling through the interface. Such has proven to be ineffective.
In each instance in the prior art, individual solar cells have been dealt with as opposed to interconnected arrays of such cells forming a complete panel of solar cells.